Croly's book was so successful because it went far beyond merely offering a new political philosophy or a collection of novel policy suggestions. It did both those things, but it also gave vitality and plausibility to that philosophy and those ideas by folding them into a narrative. It presented its assertions and prescriptions as elements in a striking retelling of the story of America. The United States was founded, Croly argued, upon three not entirely compatible tenets: a belief in the virtues of pioneer individualism, a strong commitment to limited government (especially a limited central government), and an unflagging confidence in a national ideal that he dubbed "the Promise of American life," by which he meant the steady advance of democratic values and gradual amelioration of social and economic disparities. Much of our subsequent history, in his view, can be explained as a jostling for supremacy among...